This wiki was created to distill SDC (specifically not standby) down to a set of rules in the hopes of benefiting others.

The official "rules" can be found here. In practice, some of the rules almost always apply almost, and some are more flexible.

Note there have been reports of SDC to one segment removing confirmed CPU upgrades on downline segments. If maintaining the confirmed upgrade is important then these changes should be handled with care.

Most Frequently Asked Question: When do I call/go online to make the change?
Follow this flowchart:
Question 1: Are you within 24 hours of your first originally scheduled UA(*) operated flight that you're trying to change off of? If yes, continue to question 2. If no, continue to answer 1.
Question 2: Are you within 24 hours of the first UA operated flight you're trying to change on to? If yes, continue to to answer 2. If no, continue to answer 1.
Answer 1: No, you cannot call/go online yet to make an SDC.
Answer 2: If the answer to both questions is yes, then you can call/go online to make an SDC.

General Baseline Rules

These are the general rules that usually apply to SDC's; falling outside these rules usually requires payment of a full change fee and the fare difference:

The departure time of the first segment of the old itinerary and the departure time of the first segment of the new itinerary must both be within 24 hours of the moment that the change is made.

The ultimate origin and destination airports must remain the same.

Routing can be changed (addition, deletion, or change of connection points) but the routing rules of the original fare usually apply.

All remaining unflown segments in single fared itinerary are considered together (i.e., if you're flying WWW-XXX-YYY-ZZZ, you can go to a kiosk at XXX and potentially fly XXX-AAA-ZZZ or XXX-ZZZ direct).

SDC can be done an unlimited number of times, and can therefore be used to extend a trip indefinitely (pending availability).

SDC costs $75 for GM's and Silvers - the fees is waived for Golds and up (plus any applicable fare difference, see below). *G's may also get free SDC, although there is no mention of this in the official rules.

Non-elites on the same PNR benefit from any waived fees for SDC.

No fare difference is charged so long as there is availability on all of the new segments in the originally ticketed fare class. Fare class availability in all classes often opens up after T-24 and almost always opens up at T-3 unless the plane is very full.

SDC on bulk tickets, including United Specials, is not permitted.

SDC is permitted on both domestic and international itineraries.

How to SDC

SDC can be accomplished in a few different places (in rough order of likelihood of being able to get the rules bent):

United.com “change flight” link before checking in, within T-24. (Many have reported recently that this has problems and attempts to collect the full fare difference and $200 change fee.)

"Search Other Flight Options" button during OLCI. (This has spotty availability, particularly if the change would involve paying an additional fare due to fare bucket non-availability.)

United mobile app

Kiosk

Phone

Counter/Gate (although reports are that most airport agents are not familiar with the SDC policy and may not allow you to SDC to a valid itinerary--you can try to direct them to look up GG SDC, but they often won't understand that what you're trying to do is permitted by those rules)

NB: An SDC where the original itinerary (in one direction) has three or more segments cannot be done online.

Nuances and Loose Rules

Now for the hard part: figuring out where the flexibility is in the rules. This list is compiled from the responses to people saying "Can I do X?" where X is something that is not clearly permitted by the rules. One's likelihood of success here depends a bit on the method used for the SDC.

For each item, the following codes apply:
Y = possible using any method
A = possible using an agent (phone/counter), perhaps having to HUACA
A* = mixed reports of success with an agent (phone/counter),
N* = possible only if you find a very accommodating agent
N = little or no reported success.

Change from a CPU-upgraded flight and get confirmed into available R space on the new flight: N*

Change from an instrument-upgraded flight and get confirmed into available R space on the new flight: A*

Non-elite on different PNR benefiting from Elite's waived-fee SDC: A

If you encounter issues with an agent (phone or counter/gate), you can ask the agent to reference GG SDC. Additionally, GG SDC line 61 specifically references that fare classes are supposed to level out 3.5 hours before departure, so you might be able to use that to your advantage if fare classes have not leveled out and you're trying to ask an agent to open up the correct inventory or force you onto the flight.

Don't know if this has been mentioned, but it looks as if United and Continental have aligned their SDC fees (see hyperlinks). Though it seems that the (differing) SDC time-frames have remained unaltered at both airlines. Specifically, Continental allows SDCs within 24 hours; United only allows SDCs within 3 hours of making the request.

All in all, the fee changes are good for everyone but lower tier elites. For GS, Pres Plat, 1K, Plat, Prem Exec, and Golds, SDC is free. For Premiers, Silvers and non-elites it is $75.

EDIT: these fees also apply to unconfirmed standby at the gate on both UA/CO

Last edited by boat9781; May 17, 11 at 7:00 pm.
Reason: more info, fix

They're getting closer but the CO policy is still the better version IMO. They have the full 24-hour window for changes and it is not limited to trips wholly within the USA. Still, nice to see them getting closer to a single plan.

I travel mostly on high-frequency routes and I appreciate getting home a couple hours earlier when I have the opportunity. Before the fee started I estimate 25% of my flights were standby. In my book, at least, this single change outweighs all the negatives from the merger.

It should also be noted that the CO rules for SDC are in the fare rules rather than just a published policy. This means that for CO tickets issued prior to today the old rules still apply. Gonna be plenty of confusion on this one I would imagine.

Finally! The old UA policy was the worst for midtier elites, but now we not only get standby but SDC. I'll be 1K again soon so it wouldn't have affected me too much, but I'm glad they changed it as it was unfair to the 1Ps.

Before you guys all go out of your way to praise this move, allow me to dampen the celebration a little...On CO, SDC was free BUT only possible if CO still had seats for sale in the fare bucket you had originally purchased. I can't tell you how many times I was locked out of being able to do SDC. Think about it...with today's capacity in the market and demand, especially in hub markets, how many cheap fares are still being sold close to day oif departure?

I would much prefer a system where I pay $50 and don't have to worry about fining seats in my original fare bucket. But I guess YMMV.